Three feet by 2100 would require an average sea level rise of almost 11 mm/yr, or require a steady acceleration of nearly 0.2 mm/yr² over the 85 years arriving at a rate of 18 mm/yr by 2100.

Colorado University’s Sea Level Research Group
http://sealevel.colorado.edu/
keeps the satellite records for sea level as determined from satellites and they publish data
http://sealevel.colorado.edu/files/2015_rel1/sl_ns_global.txt
which tells us:In the 11 years from 1992.9595 to 2003.9544 the rate of sea level rise was 3.5 mm/yr.and In the 11 years from 2003.9544 to 2014.9494 the rate of sea level rise was only 2.9 mm/yr.

That works out to a negative acceleration of minus -0.05 mm/yr².

Steve Case – Milwaukee, WI